Because carbon fibers have higher specific strength and specific modulus compared to other fibers, these carbon fibers are variously utilized in general industrial uses including cars or civil engineering and construction, pressure vessels and windmill blades, in addition to conventional uses such as sporting purposes or aerospace applications, as reinforced fibers used in composite materials. Thus, there is a strong demand to increase productivity or production stability.
Carbon fibers, in particular, polyacrylonitrile (hereinafter there may be cases where this is abbreviated to PAN) based carbon fibers, have been industrially produced by subjecting a spinning solution including a precursor of the above carbon fibers, namely, a PAN-based polymer to wet spinning, dry spinning or dry/wet spinning to obtain carbon-fiber precursor fibers, which are then heated in an oxidation atmosphere to be converted into flameproof fibers, followed by heating such fibers in an inert atmosphere to carbonize them.
Unlike the acrylic fibers used in clothes, the polyacrylonitrile-based fibers, which are a useful precursor of carbon fibers, are an intermediate product required for the manufacture of the final product of carbon fibers. Accordingly there is a need for polyacrylonitrile-based fibers from which carbon fibers can be produced which have superior quality and performance, and also in the course of spinning precursor fibers, the polyacrylonitrile-based fibers exhibit good stability and may be provided at low cost with high productivity upon burning to form carbon fibers, which is regarded as being very important.